For months, U.S. Marine veteran and businessman Kevin Nicholson has been laying the groundwork to enter the Republican race to challenge Democratic U.S. Senator Tammy Baldwin in 2018.

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Republicans like Nicole Schneider, Leah Vukmir, Kevin Nicholson and Eric Hovde are already lining up to run against .US. Sen Tammy Baldwin. And outside spending groups are already spending big to support -- or attack -- them. Why so soon?

Even though Nicholson, 39, has courted GOP insiders and appeared at Republican dinners, he has yet to formally enter the race and remains relatively unknown among the wider electorate.

That may be about to change with Thursday's announcement that GOP mega-donor Richard Uihlein has contributed $2 million to Solutions for Wisconsin, a newly formed super PAC that endorsed Nicholson.

"I am excited to support Solutions for Wisconsin and to strongly urge Kevin Nicholson to run for the U.S. Senate,” Uihlein said in a statement released by the group.

Uihlein, who lives in Lake Forest, Ill., is founder of the Pleasant Prairie-based Uline company. According to the Center for Responsive Politics, Uihlein and his wife, Elizabeth, contributed $23.6 million to federal candidates and other groups during the 2016 election cycle.

"I’ve never made a commitment of this significance so early in a cycle," Uihlein said in a statement released by Solutions for Wisconsin. "Kevin served our country in the Marines including combat tours in Iraq and Afghanistan, and I look forward to him serving our country again ... this time as a common sense voice for Wisconsin in the United States Senate.”

Nicholson lives in Delafield and works at ghSMART, a management consulting firm. He was a U.S. Marine captain, serving five years. According to his online profile, he has a master's degree in public administration from Harvard's Kennedy School of Government and an MBA from Dartmouth College's Tuck School of Business.

In 2000, he was president of the College Democrats of America and spoke at the party's national convention. He later shifted to Republican politics.

By going all in on Nicholson so early, and promising to contribute more, Uihlein is sending a message to others in what could be a large GOP primary field.

"I strongly encourage others to support this effort and avoid a repeat of 2012’s divisive Republican primary. I look forward to continuing to support Solutions for Wisconsin," Uihlein said.

The spokesman for the group is longtime GOP operative Chris Schrimpf, who was communications director for Gov. Scott Walker during his first term.

Others who are considering a run in the GOP primary include Madison businessman Eric Hovde, state Rep. Dale Kooyenga of Brookfield, state Sen. Leah Vukmir of Wauwatosa, Senate Majority Leader Scott Fitzgerald of Juneau and Schneider.

Baldwin campaign manager Scott Spector said: "With out-of-state millionaires and super PACs lining up to buy Wisconsin's Senate seat, Wisconsinites never have to worry about whose side Tammy is on. She has never been afraid to stand up to Washington millionaires and special interests when the best interests of Wisconsin are on the line."